Patent Document 1 discloses a cooling water circuit system including a radiator cooling water circuit, a radiator bypass circuit, a heat exchanger, a radiator downstream passage, and flow rate adjusting means. Through the radiator cooling water circuit, the cooling water flows by way of a radiator. The radiator bypass circuit bypasses the radiator. The heat exchanger is disposed in the radiator bypass circuit so as to exchange heat between the cooling water and hydraulic oil of an automatic transmission of an engine. Through the radiator downstream passage, which is connected to the radiator cooling water circuit at a downstream side of the radiator and an upstream side of the heat exchanger, the cooling water having passed through the radiator flows into the heat exchanger. The flow rate adjusting means is for adjusting a flow ratio between the cooling water flowing through the radiator bypass circuit into the heat exchanger and the cooling water flowing through the radiator downstream passage into the heat exchanger, and is disposed at a connection between the radiator bypass circuit and the radiator downstream passage.